Strategy:

Incorporate species and habitat threshold data in planning and decision making to increase the persistence and resilience of ecosystems and protect the benefits they provide to human society.

Severe storms, sea level rise, heat waves, and other climate change threats can have major impacts on species productivity and ecosystem functioning. As the effects of climate change on natural resources grow, the possibility of ecosystems experiencing major, rapid, and/or irreversible changes increases.6,8,10 In addition, climate change is expected to worsen non-climatic threats (e.g., urban development, pollution) that could cause a species or habitat to experience a major shift more quickly. Understanding how keystone species and vulnerable habitats are likely to respond to threats can help natural resource managers better prepare for and possibly avoid major, abrupt changes in ecosystems.10

Action

The concept of ecological thresholds. Shows that an ecosystem may exhibit multiple ‘stable’ states and threshold responses to stressors. Once a system crosses a threshold, it exhibits a new ‘stable’ state that may be undesirable or that may require new management and policy choices. Image modified from The Conservation of Change

What is an ecological threshold?
An ecological threshold is the point at which there is an abrupt change in the structure, quality, or functioning of an ecosystem or where external changes produce large and persistent responses in an ecosystem.10 A species threshold may disrupt aspects of the species’ population, productivity, reproduction, or habitat in response to a stressor. Such “tipping points” can lead to unwanted changes in ecosystems and may slow the recovery of ecosystems or limit their ability to achieve more resilient states following a disturbance.5 Examples of what happens when an ecological threshold is crossed can include changes in vegetation communities, marsh or forest dieback, hypoxia in aquatic systems, and rapid loss of wetlands or barrier island systems. Large shifts or disruptions to ecosystems can have major impacts for human society and can reduce ecosystem services, such as storm buffering capacity, water filtration, and recreational benefits.

How can thresholds inform decisions?
Threshold data can provide information about how a species or habitat is likely to respond to a particular threat. Beyond a critical threshold level, a species population may no longer be viable or management options may no longer be available. Natural resource managers, planners, and decision makers can use threshold estimates for particular species or habitats as management targets to help mitigate the negative effects of rising sea levels, extreme heat, and other stressors.4 A greater understanding of thresholds can help pinpoint when, where, and what type of management action to take to prevent or prepare for major shifts and disruptions to ecosystem functions. This can further help protect the valued goods and services (e.g., timber, fresh water, food) that people derive from unimpaired ecosystems.

What is threshold-based adaptive management?
Tolerance thresholds can be identified for keystone species (e.g., foundational, representative, or other focal species, such as the American Black Duck and Piping Plover) and their habitats to help manage for future change. For instance, estimating tolerance thresholds related to sea level rise can provide managers with more specific planning targets for coastal restoration as well as assess the effectiveness of restoration techniques through long-term monitoring. Threshold-based adaptive management allows managers to make ongoing adjustments to management approaches as new information on thresholds is gathered, such as through the use of ecological performance metrics and monitoring, with the goal of avoiding future threshold crossings.4

How have ecological thresholds been used in the past?
Understanding how systems are likely to respond to certain stressors helps inform restoration and management selection and design as well as long-term monitoring and evaluation. In some coastal areas, for instance, local sea level rise rates may outpace the ability of coastal systems to respond, such as where habitats are unable to migrate inland or accrete sediment (dynamic response) and inundation is likely (static response). Estimating when a salt marsh or barrier island may convert to open water in response to sea level rise can inform the type and timing of restoration techniques to better combat increasing inundation regimes.

What is threshold-based adaptive management?
Tolerance thresholds can be identified for keystone species (e.g., foundational, representative, or other focal species, such as the American Black Duck and Piping Plover) and their habitats to help manage for future change. For instance, estimating tolerance thresholds related to sea level rise can provide managers with more specific planning targets for coastal restoration as well as assess the effectiveness of restoration techniques through long-term monitoring. Threshold-based adaptive management allows managers to make ongoing adjustments to management approaches as new information on thresholds is gathered, such as through the use of ecological performance metrics and monitoring, with the goal of avoiding future threshold crossings.4

How have ecological thresholds been used in the past?
Understanding how systems are likely to respond to certain stressors helps inform restoration and management selection and design as well as long-term monitoring and evaluation. In some coastal areas, for instance, local sea level rise rates may outpace the ability of coastal systems to respond, such as where habitats are unable to migrate inland or accrete sediment (dynamic response) and inundation is likely (static response). Estimating when a salt marsh or barrier island may convert to open water in response to sea level rise can inform the type and timing of restoration techniques to better combat increasing inundation regimes.

The Sachuest Point National Wildlife Refuge in Rhode Island is using thin-layer deposition to supplement marsh accretion and support the marsh plant species Spartina patens to combat accelerating rates of relative sea level rise. Credit: US Fish and Wildlife Service.

Case study
At the Sachuest Point National Wildlife Refuge, Rhode Island, scientists have estimated the optimal elevation needed to ensure the marsh can sustain the plant species Spartina patens. In response to increasing rates of relative sea level rise, the refuge has begun layering wet sand on the marsh surface (a technique known as thin-layer deposition) using a deposition thickness threshold of 10 cm or less to support this important marsh plant.11 Refuge staff is monitoring the effectiveness of the technique using surface elevation tables to determine whether restoration needs to be revisited as sea level rise continues.11 Estimating and incorporating thresholds for Spartina patens (saltmeadow cordgrass) gives refuge staff better benchmarks for ongoing practices and long-term monitoring.

How ecological thresholds can be used:
Identifying and quantifying thresholds requires that managers know which ecosystems are most important, how stressors have affected those systems in the past, and how those stressors may change in the future. While a single stressor may not have much of an impact on an ecosystem, multiple or repeated stressors can have cumulative impacts on an ecosystem that could lead to a threshold crossing. As natural resources continue to be impacted by a changing climate and increased pressures from human activities, planning for and anticipating surprises can be beneficial.3 In addition, establishing threshold criteria in the early stages of a project can increase support for more proactive and preventative management moving forward.

There are no universal early warning indicators of a threshold crossing, as they are often species- and ecosystem-specific. However, there are a variety of tools and information to support threshold-based adaptive management.4

Activities that have been associated with effective and successful threshold-based management include:

Routine monitoring to reduce uncertainty

Incorporating thresholds into decision making

Managing threats at the appropriate spatial scale4

1. Use routine monitoring to reduce uncertainty
Long-term historical datasets combined with routine monitoring can provide insight on how climate drivers are likely to affect different features of an ecosystem. Practitioners can use historical data to understand how an ecosystem has been affected by and responded to different stressors in the past. This reduces uncertainty by providing some evidence for how future stressors may impact these ecosystems. In addition, monitoring using quantitative measurements to detect changes in an ecosystem function (known as ecological metrics) can help detect important drivers of change and possible ecological threshold crossings. Ecological changes indicative of threshold crossings could include changes in community composition and the loss or reduction of population connectivity, function, or biodiversity.4 Increasing variability may also be an early indication of system instability and possible threshold change.10

2. Incorporate thresholds into decision making
Research quantifying ecological thresholds is limited but increasing.10 When numeric thresholds are available, they can help guide planning and action. Cost-benefit analysis is a tool that can help link ecological thresholds to decision making.7 Once a known ecological threshold is approached or crossed, management costs are likely to rise in order to account for the adverse impacts or to restore the habitat to a more resilient state. In many cases, acting preemptively to avoid crossing a threshold can greatly reduce management costs and protect the human benefits of an ecosystem.7 Matching thresholds to management decisions or alternatives using cost-benefit analysis can help to ensure that ecosystem services are maximized while management costs are minimized.7

A cost-benefit analysis may be most effective by:

Identifying what the ecological threshold of concern is and how different stressors may drive the system toward the threshold;

Estimating the costs and benefits associated with the ecosystem services in the area of concern; and

Estimating the costs of mitigating the stressor.

Estimating each of these parameters can be difficult and expensive; however, such an analysis can yield more effective and positive results.7

3. Manage threats at the appropriate spatial scale
Different patterns of change may be observed at local, regional, and global scales. Drivers of change can be global in nature, such as sea level rise, or local in nature, such as point-source pollution (a single identifiable source of pollution). Designing and implementing effective management actions requires acting at a scale appropriate for addressing the driver and the ecosystem response.4

Additional challenges to consider:

Avoiding the crossing of a threshold may require rapid decision making, which might be difficult under existing management and political structures. However, prevention is often less costly and time consumptive than restoring an ecosystem that has crossed a critical threshold.

Proactive management to avoid threshold crossings before they occur can be resource and information intensive and contain unavoidable uncertainty.7 However, informed management decisions that account for thresholds can yield more desirable outcomes over the long term.

A habitat or species response to a management action can be much slower than its response to a stressor. Adaptive management actions that support timely and responsive actions can better support management outcomes.4

Partnerships are key. A single organization may not have the resources alone to adequately address climate drivers and their impacts. Coordination and cooperation among institutions with shared goals builds capacity and increases the overall chances of success.

Despite the inherent complexities in quantifying thresholds, managing for ecological thresholds can enhance management outcomes and increase the resilience of ecosystems to future threats. Practitioners can begin to manage for threshold changes by using quantitative targets where possible, conducting ongoing and responsive monitoring efforts, and implementing appropriately-scaled management actions.4 While these methods may not help to quantify thresholds, they can build capacity for considering, incorporating, and avoiding thresholds.

3. Dziegielewska, D.. 2012. Within the Boundaries: Management Options for Conservation Lands in an Era of Climate Change. Report developed for U.S. Fish and Wildlife Service by the Wisconsin Department of Natural REsources and the University of Wisconsin-Madison.